Variable diameter sprockets are well known in the art. For example, U.S. Pat. Nos. 4,030,373; 4,832,660; 5,041,061; 5,013,284; 5,094,653; and 5,104,357 all naming Hamlin Leonard as an inventor, each describes variable speed drive systems for bicycles provided with variable diameter pulleys. As is well known, by modifying the diameter of the variable diameter pulley, the ratio of the front to rear pulleys is modified without requiring a cluster of sprockets.
The systems described by Leonard do not use a conventional bicycle chain but instead use a flexible driving means under the form of a V-belt. The different mechanisms described by Leonard have the same common drawback that the diameter changing mechanisms provided to modify the diameter of the pulley are generally complex.
The use of a V-belt is also a drawback since it must usually be heavily tensioned to prevent it from slipping from the pulleys. It is also to be noted that adverse temperature conditions, such as rain, may nevertheless cause the V-belt to slip in the pulleys.
To overcome the drawbacks described hereinabove with respect to the systems proposed by Leonard, the applicant describes in an international application published under publication number WO99/43539, a variable ratio drive system using a conventional chain and a simple sprocket diameter changing mechanism. It has been found however, that the friction between the sprocket diameter changing mechanism and the sprocket portions is somewhat detrimental to the performance of the drive system.